The present invention relates to the field of precision surface treatment, in particular to abrasive materials used in such treatments, and to the method of manufacturing the abrasive materials.
A conventional abrasive material for precision surface treatment usually consists of a powder of free abrasive grains of micrometer-submicrometer size which are made of materials of high hardness. The abrasive grains are commonly embedded within a polishing paste or fixed within a layer applied to some other form of carrier.
Flexible abrasive materials are used to effectively polish objects having an irregular surface or shape. Such flexible abrasive materials may be in the form of abrasive belts which have strength and flexibility characteristics sufficient to provide tight contact between the surface of the abrasive belt and the irregular surface of the object being polished.
For precision surface treatment it is preferable to use diamonds as the abrasive grains since diamonds possess the highest hardness value of all known substances. Unfortunately natural diamond resources are extremely limited and artificial diamond crystals are very difficult to produce. The difficulties in obtaining artificial diamonds are primarily due to the high temperature and pressure processing requirements. In view of this fact, the search for new, highly efficient abrasive materials suitable for the precision treatment of surfaces, as well as for the processes which would permit such abrasive materials to be manufactured, is a continuous, ongoing effort.
Diamond-like carbon ("DLC") is an amorphous carbon material deposited as a coating using either plasma enhanced physical vapor deposition (PVD) techniques or chemical vapor deposition (CVD) techniques. These materials are called `diamond-like` because their mechanical, optical, and electrical properties can be very close to the properties of natural diamond. Diamond-like has become a standard term of art. For example, see the Proceedings of the First International Symposium on Diamond and Diamond-Like Films, The Electrochemical Society, Vol. 89-12 (1989).
Many methods of coating diamond-like carbon films onto particulates, surfaces, and fibers are known. For example, gaseous-phase plasma spraying, electron-cyclotron/magnetic field assisted chemical vapor deposition ("CVD"), and other CVD processes. Common disadvantages to all CVD methods are that they are generally inefficient, they must be carried out under reduced pressure with the application of microwave energy, and the surface to be coated must be able to withstand temperatures of at least 200.degree. to 400.degree. C. More importantly, the films produced by CVD methods yield diamond-like microstructures having a significant number of C-H bonds. For this reason, materials formed by these methods are often termed "hydrogenated diamond-like carbon" or "HDLC." Articles made by incorporating abrasive particles coated with HDLC are insufficiently flexible for most purposes unless high cost thread-like fibers coated with HDLC are employed. Less expensive and more flexible articles may be made using variations of CVD, but still result in the formation of undersized HDLC.
Thin films of HDLC are undesirable because they are dielectric or semiconductors, and thus accumulate static electricity. This property is deleterious to any abrading action since dust and other particulates accumulate and adhere to the surfaces of the abrasive material. The presence of hydrogen in HDLC thin films also limits the hardness of the films.
Physical vapor deposition ("PVD") processes for depositing diamond-like carbon ("DLC") films are known. Japanese Kokai Patent No. H2-266926 describes an abrasive tape formed from a PVD layer deposited on a plastic substrate. A disadvantage of the process is that it cannot produce an abrasive layer with a microhardness exceeding that obtained for HDLC.
Ion beam, super high vacuum processes are known which deposit carbon ions from an ion beam. Although such techniques produce carbide-type films of high hardness, they require super high vacuum, have low efficiency, and are technically complex.
A more simple method for obtaining vacuum condensates of various materials, including DLC, is known in the art. This method consists of creating a plasma of highly ionized carbon and periodically depositing portions of the plasma onto the surface of a substrate. These portions are obtained by the pulse-laser evaporation of graphite in a vacuum, under a pressure for the residual gases of about 10.sup.-6 Torr, with the subsequent intensification of plasma formation by means of a vacuum electrical-arc discharge. (cf. H. J. Scheibe and P. Siemroth, "Film Deposition by Laser-Induced Vacuum Arc Evaporation," in IEEE Transactions on Plasma Science, Vol. 18, No. 6, December 1990, 917-922).
Due to the short-term nature of the deposition process with the pulse-type methods described above, it becomes possible to obtain vacuum condensates on substrates having a low thermal destruction temperature, e.g. on plastic substrates. However, these condensates generally demonstrate undesirable dielectric properties. (cf. J. Krishnaswani, et al., "Laser and Plasma Enhanced Deposition of Diamond and Diamond-Like Films by Physical and Chemical Vapor Deposition Technique," in SPIE, vol. 1190, Laser-Optical Processing of Electronic Materials, 1989, p. 109-117).
Other disadvantages associated with the above described pulse method are the high costs of lasers, the instability of the excitation of the vacuum-arc discharge with the pulse-laser evaporation of graphite, and the rapid dusting-over of the window used for the introduction of laser radiation into the vacuum chamber. The last effect is especially noticeable when the process is being carried out at a high rate.
The cathodic arc process, also known as vacuum arc or electromagnetic erosive plasma accelerator, is known to produce DLC coatings that are harder than either sputtering or laser ablation processes. This discharge occurs in a vacuum environment when a sufficient current, greater than about 50 amps, is passed between electrodes. Once the discharge is started the current from the cathode passes through a small spot called the arc spot, estimated to be about 10 microns, on the cathode surface. The extraordinary current densities of over a million amps per square centimeter causes a flow of ions and macroparticles to be ejected form the arc spot. The fraction of ions that are produced can be quite high and is higher for higher melting point cathodes, for example carbon discharge have ion fractions of over 90-95%.
The vacuum or cathodic arc discharge can be powered either by a DC power supply, frequently operating in a current controlled mode, or from a charged capacitor bank. The latter is often referred to as pulsed cathodic (or vacuum) arc discharges or as an electromagnetic erosive plasma accelerator.
There are several methods to initiate the arc discharge; a mechanical `striker` may be used to strike the arc, similar to a welder, or several different types of secondary discharges (e.g., a spark, puff of gas plasma, or thin film ablation) may be directed into the interelectrode region.
The high ion fraction of carbon cathodic arc discharges makes this process an ideal method to produce superhard and non-hydrogenated DLC coatings.
U.S. Pat. No. 5,075,848 describes a method for the deposition of coatings from plasma flows by means of a periodically generated pulse electromagnetic erosive plasma accelerator, operating on the basis of a high-current vacuum electrical arc. This method consists of positioning the substrate within the deposition chamber, evacuating the chamber, and then injecting portions (doses) of the initiating plasma into the electrode gap of the accelerator. Each of the plasma portions in question is obtained by passing a firing-current pulse between an consumable electrode and a firing electrode. The firing electrode is located within the consumable electrode, and installed in such a manner that a gap forms between the two of them. In operation, the gap must first be electrically broken then high-current vacuum electric arc discharges are excited in the electroerosive plasma being emitted from the plasma-forming surface of the consumable electrode. This process results in the generation of pulse flows of the accelerated electroerosive plasma of the substance being precipitated by means of the accelerator. The plasma flows are directed towards the surface of the substrate, forming a vacuum condensate on the surface of the substrate by the deposition of the generated flows of plasma.
Such a method makes it possible to obtain a thin film vacuum condensate of various materials, including DLC, with good adhesion even if the substrate surface is maintained at a low temperature during condensation. However, the separation of the pulse flows, which in this process is carried out with the use of a curvilinear solenoid connected in series with the electrode gap of the plasma accelerator, appears to be insufficiently effective. Furthermore, an extremely high pulse pressure (not less than 5 to 10 kV) must be applied to the vacuum gap between the eroding and firing electrodes in order to obtain each portion of the initiating plasma. During this process the flows of the generated electroerosive plasma receive a considerable number of particles having an extremely high energy. This is highly undesirable for many vacuum condensates, especially thin DLC films. Finally, it should be noted that the periodic high-voltage breakdown of the above-described vacuum gap constitutes a rather complicated technical problem. Such a process is characterized by low reliability and low stability.
For all of the reasons previously discussed the mass production of high-quality vacuum condensates, in particular carbon, is very desirable. It is especially desirable to have a high level of reproducibility of condensate composition and microstructure as well as other properties. Although the above identified methods make it possible to apply, in a vacuum, coatings of various materials including DLC, they are still not suitable for manufacturing carbon-based abrasive materials.
It is an object of the present invention to provide a superhard abrasive material, preferably carbon-based, characterized by high polishing ability, applicable to the treatment of objects made of various materials, and being generally suitable for a variety of applications. It is another object of the invention to make it possible to produce the abovementioned abrasive material both in the form of free abrasive and as a bound abrasive attached to a flexible or rigid carrier. Still another object of the invention is to provide a carbon-based superhard abrasive material with a low surface electric resistivity. Another object is to provide an improved, highly reliable and efficient method of forming a carbon-based vacuum condensate abrasive material using pulse-type electromagnetic erosive plasma accelerators. It is an object of the invention to provide this method in a way which ensures the reproducibility of the material's characteristics, and which would allow the use of low thermal destruction temperature carriers.
Another object of the invention is to provide a method which allows the industrial-scale production of abrasive materials comprising superhard vacuum condensates, preferably carbon-based, with characteristics superior to those obtained by conventional methods of continuous vacuum vapor deposition, and with a corresponding superior quality.